Fixations and Visual Attention Flashcards
T/F The eye is NEVER motionless
true
What is the functional fovea area?
+/- 30’ of arc discrete area on the retina within which images are held
What happens to the discrete area on the retina the longer fixation is held?
longer fixation=larger area
What are three types of movements (subconscious)
tremors, drifts, and micro saccades
What is a tremor?
constant, high frequency 30-100 Hz, amp 20” of arc, no VA change
What is a drift?
low velocity <0.5 Hz, crosses 15 cones/sec, irregular movement
What is a microsaccade?
1-2/sec, corrects drift, large dynamic overshoot component, error correcting, amp of 5’ of arc, amplitude dependent relation
Which movement is always binocular?
micro saccades
Which movement do eyes perform most of the time?
drifts (95%)
Which movement is superimposed over the other two?
tremors
What factors impact movements?
peripheral gaze direction increases probability of movement, darkness: less steady and more drifts/correcting saccades
During normal fixations, what is the relationship between drift and microsaccades?
Drifts and micro saccades move in the opposite direction and drift is error producing and occurs 95% of the time while micro saccades are error correcting
What two things make drifts worse?
greater gaze angle and darkness– evidence of leaky neural integrator
What improves accuracy of fixations?
visual feedback
Even w/o visual feedback, deviation is typically only a few degrees, what does this suggest?
use of EOM proprioception and tactile info from lids
What are the three main types of abnormal fixation movements?
slow drift, saccadic intrusion, and nystagmus
What is a slow drift?
found with functional amblyopia, amp 1 degree, velocity <3 deg/sec, irregular, slow frequency <0.5 Hz
T/F VT will NOT fix a slow drift
false
What is a saccadic intrusion
large fixational saccade that interrupt attempts at fixation; three kinds= square wave jerks, macro square wave jerks, and macro saccadic oscillations
What is an aberrant tremor?
seen w/ EEG or coma patient because other movements usually cover them; baseline state of brainstem
What is a square wave jerk?
away from the target and then back 200 msec later; dx cerebellar disease if freq/amp increases
What is a macro square wave jerk?
larger, more frequency duration off target 100 msec; dx cerebellar disease and multiple sclerosis
What is a macro saccadic oscillation?
saccadic sequence increasing then decreasing amplitude to either side of a target; ALSO dx cerebellar disease
Which of the abnormal fixation movements might you see in clinic?
macro square wave jerks
T/F fixation in amblyopia improves with feedback
true
Explain the feedback received by amblyopic patients in the study
goggles w/ a computer program track fixation, and when the eye leaves the target, the computer makes an auditory stimulus until focus is corrected– allowing the patient to learn to control fixation
What happens during a fixation?
EOMs keep the eyes still with respect to the visual environment as the head/body moves
What two systems promote visual stabilization?
VOR vestibulo-ocular and OKN optokinetic
What prevents image degradation?
micro saccades and the troxler effect
What is true of the scan path of a photograph under free exploration?
most of the focus is on the face and other objects
What was the Yarbus free exploration study?
subjects typically focus on faces and objects; under command to judge material status look at clothes and servants; under command to judge age differences look at faces and height
What is true of an expert viewer as compared to a novice?
Expert has fewer fixations, shorter duration, less random search pattern of fixations– uses vision to derive meaning
What is the primary purpose of the visual system?
to derive meaning and the direction of action
What is a “consequence” of visual attention?
selection of an area to pay attention to will cause the omission of some other things (figure vs ground) due to limited scope of attention
What happens to things we focus on during visual attention?
they get amplified in the system, allowing for persistence of their image for further inspection by higher order process
Most visual problems are problems of ____ not _____
omission not commission
What are the components of the visual attention system?
attention window and visual buffer, encoding processes, memory, information look up, and attention shifting
What is inside the attention window?
figure
What is in the visual buffer?
ground
____ emerges from ____
figure, ground
What is the figure processed by?
object properties–what
What is the ground processed by?
spatial properties– where
What does associative memory do?
accesses previous info and gives context
What happens with attention shifting?
restart the process, does not require an eye movement
What steps does attention/shape shifting entail?
disengage, move, engage
T/F we have to have a stimulus to shift attention
false, we have volitional control, we can also inhibit a shift of attention to a stimulus
Which steps of shape shift change the size of the attention window?
disengage and move
What step do patients with autism have trouble with?
engage
What step do patients with parkinsons have trouble with?
disengage
What step do myopes have trouble with?
disengage
What step do ADHD patients have trouble with?
inhibition
What is sensory memory?
the what system
What is motor memory?
the where system
Object recognition in human vision is provided by:
separate but simultaneous processing and representation of what and where information at high levels of the visual system
Frame of reference is attached to…
the basic feature at each fixation point and includes what and where data
Mechanisms of visual attention use ____ info to direct sequential image processing (what to do next)
where– context and planning
Mechanisms provide comparisons of ___ and ___ object features in each fixation
current and expected
How do we look at a face?
One eye to the other then down the nose and around the face
